I am rendering a tracked scene where I added only a shadow catcher ground, and a few storm trooper animations. My PC can render with it's CPU (i5) 16x16 the storm trooper model with motion blur almost the same speed it renders the transparent areas. This causes lots of render time of pointless areas of each frame. (A lot of the transparent areas do have light shadows)
Is it possible to render only the parts where shadows and models exist, and "skip" the transparent areas, or go through rendering the transparent areas quicker?